1. Field of the Invention
The present invention relates to an improved method of processing color image data for maintaining edge quality while printing on a color ink jet printer.
2. Description of the Prior Art
Liquid ink printers including inkjet printers deposit black and/or colored liquid inks which tend to spread when the ink is deposited on paper as a drop, spot, or dot. A problem of liquid ink printers is that the liquid inks used have a finite drying time, which tends to be somewhat longer than desired. Bleeding tends to occur when the drops are placed next to each other in a consecutive order or in a cluster of dots within a short time. Bleeding, spreading, and feathering causes print quality degradation including color shift, reduction in edge sharpness, and solid area mottle which includes density variations in said areas due to puddling of inks. Intercolor bleeding occurs when ink from one color area blends into or bleeds with ink from another color area. Intercolor bleeding is often most pronounced where an area of black ink (relatively slow drying) adjoins an area of color ink (relatively fast drying); however, intercolor bleeding can occur at the interface between areas of any color inks having substantially different properties such as dry time or permeability.
To solve this problem, many solutions have been proposed. In U.S. Pat. No. 6,183,062 entitled “Maintaining Black Edge Quality in Liquid Ink Printing” and assigned to Xerox Corporation, Curtis et al. teach a method for maintaining edge quality between black ink and colored ink, which is incorporated herein by reference.
Please refer to FIG. 1. FIG. 1 is a flowchart illustrating printing color images according to the prior art. Steps contained in the flowchart will be explained below.    Step 10: Start the process for printing a color source image;    Step 12: Perform a color conversion operation on the source image. This conversion typically involves converting red, green, and blue (RGB) colors into cyan, magenta, yellow, and black (CMYK);    Step 14: Convert the color image into a plurality of halftone images. For example, a color plane is produced for each of the CMYK colors, producing four halftone images;    Step 16: Pixel altering processing is performed on each of the halftone images;    Step 18: The altered halftone images are printed; and    Step 20: End.
To reduce intercolor bleeding, the prior art carries out a process that operates to detect black/color interfaces where intercolor bleeding is likely to occur and to modify the pixels that are to be printed near the borders of the interfaces. The process comprises three general steps: identifying an interface between a black area and a color area; modifying the pixel pattern in a black border region in the black area; and modifying the pixel pattern in a color border region in the color area. Please refer to FIG. 2. FIG. 2 shows a flowchart illustrating the prior art method for altering pixels in the halftone image for reducing intercolor bleeding.
Step 16a identifies an interface between a black area and a color area. In one embodiment, described in more detail below, step 16a collects statistics for pixels within an X×Y window filter to identify an interface and determine if a given pixel is within either border region. However, step 16a can use any number of known techniques including, but not limited to, masking, look-up tables, edge detection filters, etc. to identify a black/color interface. A discussion of edge detection filters can be found in U.S. Pat. No. 5,635,967.
Step 16b defines a width N of the black border region near the black/color interface identified in step 16a. The number of pixels N comprising the black border region should be large enough to reduce intercolor bleeding at the border and small enough to minimize the formation of additional printing artifacts that would likewise reduce image quality. Similarly, step 16c defines the width M of the color border region near the interface. As with the selection of black border region, the width M of the color border region should be selected to reduce intercolor bleeding while minimizing the addition of other printing artifacts.
When defining the width of the border regions consideration may be given to such factors as the position of the border regions, the type of image (e.g., text, line art, graphics, pictorial, etc.), the width of each border, how the pixel pattern within a border will be modified, the print medium used, ink composition, etc. Each of the border regions beneficially are positioned to abut the interface; however, it is understood that the border region need not abut the interface. The total width of the border regions at an interface should be selected to reduce intercolor bleeding at an interface and minimize the formation of additional printing artifacts. Optimum values for border width can be identified through calibration and image analysis studies. The width of the black border is preferably between 0 and 350 μm, and the width of the color border is preferably between 0 and 200 μm. Beneficially, for a 300 dpi ink jet, the width of the N pixel black border is selected from 0 to 4 pixels, and the width M of the color border is defined to be from 0 to 2 pixels.
Steps 16d and 16e modify the pixel pattern within the N-pixel black border and M pixel color border regions, respectively. A number of methods exist to modify the pixels or pixel pattern within the border regions. One method of modifying the pixel pattern within a border region replaces selected pixels with a predetermined combination of separation pixels. The replacement operation effectively turns off the separation pixel this is being replaced and turns on the separation pixel(s) replacing it. The replacement of pixels is sometimes referred to as “substitution” or “replacing”. An example of a substitution operation is illustrated in FIG. 3. In FIG. 3, window 40 shows a 5×5 block of composite pixels along a yellow/black interface. Window 42 shows the pixel block of window 40 after a substitution operation wherein within a 2 pixel border (columns 44 and 46) every other pixel in the black separation is turned off and replaced with alternating cyan and magenta pixels in the composite image.
Another method of modifying a pixel pattern removes (turns off) selected pixels in one or more separations from the composite image. This removal of pixels from separations is sometimes referred to as “thinning” or “reducing”. FIG. 4 illustrates an example of a thinning operation wherein window 50 is a 5×5 pixel block of composite pixels along a black/color interface and window 52 shows the same image block after thinning. The thinning operation removes (turns off) all color separation pixels from every other pixel in column 54 and removes yellow separation pixels from every other pixel in column 56.
A thinning operation can also be used to reduce the ink coverage in a multiple drop per pixel printer. Briefly, in a multi-drop per pixel printer small ink drops are often used to produce good tone transitions in graphical and pictorial images. However, the size of these drops are not large enough to produce a solid area fill or saturated colors using only one drop per pixel, thereby reducing the color saturation value for that pixel. Thus, the printer typically requires greater than 100% coverage, that is, multiple drops per separation pixel to obtain solid area fill. In FIG. 5 window 60 illustrates a 5×5 pixel area along a black/color interface wherein the black region comprises 150% coverage (i.e., an average of three drops for every two pixels). Window 62 shows the same image area as window 60 after a thinning operation to reduce the drop coverage to approximately 100%, ie., an average of one drop per separation pixel. In window 62, column 64 illustrates a thinning operation that reduces all two drop pixels to one drop pixels. Columns 66 and 68 illustrate a thinning method that removes approximately half of the two drop pixels.
As shown in steps 14 and 16 of the flowchart of FIG. 1, the prior art method involves first converting the source image into halftone images, and then altering the pixels of the halftone images in order to reduce intercolor bleeding. Unfortunately, if the halftone images have a higher resolution than the source image, many extra calculations and extra memory are required to alter pixels on the halftone images as compared to altering the pixels on the source image.